1. Field of the Invention
The present invention relates to fusaric acid resistant genes cloned from fusaric acid decomposing or detoxifying microorganisms, plasmids having the genes, host cells transformed by the plasmids, proteins having amino acid sequences specified by the base sequences of the genes, respectively and capable of participating in the decomposition or detoxification of fusaric acid and production processes of the proteins, and utilization of the genes and proteins.
2. Description of the Related Art
Wilting disease which causes plants to wilt or droop to die is a serious disease for cucurbitaceous plants such as cucumber, water melon and melon and solanaceous plants such as tomato and egg plant. Wilting disease is induced by soil-inhabiting fungi of the genus Fusarium. Soil sterilization and the avoidance of continuous cultivation of a particular crop are the only measures effective for the control of wilting disease caused by fungi of the genus Fusarium. These measures are however difficult to practice for most farmers from both technical and economical viewpoints, yet any effective control method which may substitute for these measures has remained unavailable.
Fusaric acid (5-n-butylpicolinic acid) is known to be a toxin which is produced by plant pathogenic fungi of the genus Fuarium and non-specifically acts on a wide variety of plant cells [Wood, R.K et al., "Phytotoxins in plant diseases", Academic Press, New York (1972); Durbin, R.D., "Toxins in plant diseases", Academic Press, New York (1982); Gaumann, E., Phytopathology, 47, 34 (1958)]. Although the mechanism of action of fusaric acid has not been fully elucidated, fusaric acid is known to exhibit, for example, strong toxicity on tomato callus cells and tomato stems and leaves, to increase the plasma membrane permeability of tomato cells whereby the exudation of the tissue fluid is induced to accelerate death [Matsuo, Hayami et al., "Sakumotsu no Fusarium Byo (Fusarium disease of crops)", Zenkoku Noson Kyokai (National Association of Agricultural Communities)], to inhibit germination and rooting of barley [Foy, C.L. and Change, I., Adv. Pestic. Sci., 3, 499 (1979)], and to show proliferation inhibitory effects against fungi and bacteria [Kalyamasundram, R., Plant Dis. Probl., 1, 142 ( 1970)].
On the other hand, there are some varieties of tomato which exhibit resistance to diseases caused by fungi of the genus Fusarium. Fusaric acid is metabolized and converted to N-methylfusaric acid amide in plant tissues of these resistant tomato varieties. A variety with stronger fusaric acid resistance shows greater fusaric acid converting action. As a matter of fact, plants having such resistance to fusaric acid have been found to be resistant to wilting disease caused by Fusarium oxysporium [Shahin, E.A. and Spirey, R., Theor. Appl. Genet., 73, 164 (1986); Toyoda, H., et al., Phytopathology, 78, 1307 (1988)].
Fusaric acid is known to exhibit not only wilting toxicity against plants but also toxicity against microorganisms such as bacteria. As fusaric acid resistant microorganisms capable of growing on or in a culture medium containing fusaric acid, the present inventors isolated several types of microorganisms from soil. The isolation methods, mycological characteristics, fusaric acid resisting mechanism, etc. of these fusaric acid resistant microorganisms are disclosed in detail in Japanese Patent Application Laid-Open Nos. 198974/1988 and 198987/1988. One of the above fusaric acid resistant microorganisms has been identified as Pseudomonas cepasia. This bacterium has been found to have fusaric acid decomposing ability and also to be able to grow using fusaric acid as a sole carbon source, in other words, to have fusaric acid assimilating ability.
It may hence be considered feasible to avoid development of an infectious disease by a fungus of the genus Fusarium-such as wilting disease which is very difficult to control-provided that fusaric acid which is the non-specific toxin produced by the fungus of the genus Fusarium can be decomposed and eliminated. For this purpose, decomposition or detoxification of fusaric acid by one of the above fusaric acid resistant microorganisms can be an effective means. It is however very difficult to bring such a fusaric acid resistant microorganism into contact with fusaric acid produced within a plant due to infection by a fungus of the genus Fuarium.
As a means for overcoming such a difficulty, it has been a practice to some extent to breed a plant, which can decompose or detoxify fusaric acid, by isolating a fusaric acid resistant gene from one of the above fusaric acid resistant microorganisms and then introducing the gene in a plant. Plants having such fusaric acid resistance can be used as effective means for the control of wilting disease caused by a fungus of the genus Fusarium. Knowledge on the fusaric acid resistant gene and its structure is indispensable to make use of such a genetic engineering technique.